Explosion of a hardening vessel for AAC (autoclaved aerated concrete)

Abstract

A vessel for the hardening of aerated concrete exploded after modification. The modification was implemented by welding additional barrels to extend the vessel. Two and a half years after the modification (about 500 load cycles) the vessel exploded showing a longitudinal crack starting in one of the new segments. After the modification the dimensions of the vessel were 2000mm diameter and 31,000mm length, operational pressures ranged from 0.3 to 16bar. The material of both the old and new parts of the vessel and the properties of the welds showed no deviations from the specifications. The geometry of the longitudinal weld showed a roofing deviation from circularity. Fractographic investigations showed a crack of 1.6m length following the longitudinal weld which developed over long time caused by corrosion fatigue. At the time of the explosion, the wall thickness was reduced from 13.5mm down to about 2mm leading to longitudinal cracking of the vessel throughout the new parts followed by a circular crack which resulted in a complete separation of the pressurized vessel. The crack development is shown by optical and SEM-micrographs.Calculations of nominal stresses show that the geometrical deviations by linear misalignment and roofing cause a stress (tension load) increase of about 4.8 of the mean tensile load value. Adjacent to the weld this causes a local tension overload; in combination with operation conditions (overheated steam of 203°C, cyclic operation) this causes corrosion fatigue due to cracking of corrosion layers. The crack formed by this mechanism caused a remaining wall thickness of less than 2mm followed by a ductile residual fracture.